1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly it relates to a liquid crystal display device implementing in-plane switching (IPS) in which an electric field is generated in a plane that is parallel to the substrate of the device.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. The long thin shapes of the liquid crystal can be aligned to have an orientation in a specific direction. The alignment direction of the liquid crystals can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy of the liquid crystal, the refraction of incident light depends on the alignment direction of the liquid crystal molecules. Thus, by properly controlling an electric field applied to a group of liquid crystal molecules in respective pixels, a desired image can be produced by diffracting light.
There are many types liquid crystal displays (LCDs). One type of LCD is an active matrix LCD (AM-LCD) that has a matrix of pixels. Each of the pixels in an AM-LCD has a thin film transistor (TFT) and pixel electrode. AM-LCDs are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they have the characteristics of light weight, thin profile and low power consumption. The typical liquid crystal display panel of an LCD device has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors, and pixel electrodes.
The operation of an LCD device is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. More particularly, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of liquid crystal molecules is properly controlled in each pixel of a group of pixels, incident light is refracted along the alignment direction in a plurality of pixels to display image data. Thus, liquid crystal molecules in the pixels function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
In a conventional LCD device, the pixel electrode and common electrode are positioned on the lower substrate and upper substrate, respectively. Thus, a longitudinal electric field is induced between the lower and upper substrates of a conventional LCD device. This longitudinal electric field is perpendicular to the lower and upper substrates. However, conventional LCD devices having the longitudinal electric field have a drawback in that they have a very narrow viewing angle.
To solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) device has been proposed. The IPS-LCD device typically includes a lower substrate on which a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates. A detailed explanation about operation modes of a typical IPS-LCD panel will be provided in reference to FIG. 1.
FIG. 1 is a cross-sectional view illustrating a concept of a related art IPS-LCD panel. As shown in FIG. 1, upper substrate 10 and lower substrate 20 are spaced apart from each other, and a liquid crystal layer 30 is interposed therebetween. The upper substrate 10 and lower substrate 20 are often referred to as a color filter substrate and an array substrate, respectively. A common electrode 22 and a pixel electrode 24 are located on the lower substrate 20. The common electrode 22 and pixel electrode 24 are parallel alignment with respect to each other. A color filter layer (not shown) is positioned in an area of the surface of the upper substrate 10 that corresponds to an area between the pixel electrode 24 and the common electrode 22 of the lower substrate 20.
A voltage applied across the common electrode 22 and pixel electrode 24 produces an electric field 26 through liquid crystal molecules 32. The liquid crystal molecules 32 have a positive dielectric anisotropy, and thus orient to have an alignment which is parallel with the electric field 26. When no electric field is produced between the common electrode 22 and pixel electrode 24, i.e., “off state”, the longitudinal axes of the liquid crystal (LC) molecules 32 are aligned in a direction that is parallel to and form a definite angle with the common electrode 22 and pixel electrode 24. For example, the longitudinal axes of the LC molecules 32 are arranged in a direction parallel with both the common electrode 22 and pixel electrode 24. In contrast, when a voltage is applied across the common electrode 22 and pixel electrode 24, i.e., “on state”, a lateral electric field 26 parallel to the surface of the lower substrate 20 is produced because the common electrode 22 and pixel electrode are on the lower substrate 20. Accordingly, the LC molecules 32 are re-arranged to bring their longitudinal axes into coincidence with the electric field 26. Since the LC molecules switch directions while maintaining their longitudinal axes in a plane perpendicular to the direct viewing direction of a display, in-plane switching can permit a wide viewing angle for a display device. The viewing angles can range from 80 to 85 degrees in up-and-down and left-and-right views from a line vertical to the IPS-LCD panel, for example.
FIG. 2 is a plan view illustrating one pixel of an array substrate according to a related art IPS-LCD device. As shown in FIG. 2, a gate line 40 is transversely arranged across the figure and a data line 42 is disposed substantially perpendicular to the gate line 40. A common line 50 is also transversely arranged across the figure parallel with the gate line 40 and is spaced apart from the gate line 40. The gate line 40, the common line 50 and the data line 42 define a pixel region P on the array substrate. A thin film transistor (TFT) is disposed at a corner of the pixel region P near the crossing of the gate line 40 and data line 42.
In each pixel, three common electrodes 44 extend perpendicularly from the common line 50, and two of the common electrodes 44 are disposed next to the data line 42 and the data line of another pixel, respectively. A pixel connecting line 48 that electrically connects to the TFT T is disposed next to the gate line 40 and is parallel with the gate line 40. Pixel electrodes 46 extend perpendicularly from the pixel connecting line 48. Each of the pixel electrodes 46 is disposed between two of the common electrodes 44 and are parallel with the data line 42. Each of areas “I” between the respective common electrodes 44 and the respective pixel electrodes 46 is defined as a block where the liquid crystal molecules are re-arranged by an electric field. As shown in FIG. 2, there can be four blocks in one pixel.
The IPS-LCD device shown in FIG. 2 re-arranges and operates the liquid crystal molecules using an electric field generated that is parallel with the array substrate. Thus, the IPS-LCD device can provide a wide viewing angle as opposed to an LCD device using an electric field that is perpendicular to the array substrate. Recently, modifications to the IPS-LCD device have been researched for further increasing the viewing angle.
FIG. 3 is a plan view of an array substrate having multiple domains according to another related art IPS-LCD device. Some detailed explanations, especially those previously explained in reference to FIG. 2, will be omitted with regard to FIG. 3 to prevent duplicate explanations. As shown in FIG. 3, a pixel connecting line 58 is disposed over a common line 60. Common electrodes 54 and pixel electrodes 56 extend in an up-and-down direction from the common line 60 and pixel connecting line 58, respectively. Both the common electrodes 54 and pixel electrodes 56 have a zigzag shape with plural bent portions that alternate with each other. However, corresponding portions of the common electrode 54 and pixel electrodes 56 are parallel to each other. The zigzag shape defines the multiple domains in the pixel regions that are symmetrical to the bent portions of the common electrode 54 and pixel electrode 56. This zigzag shape with multiple domains further improves the viewing angle as compared to the straight shape shown in FIG. 2.
Moreover, the pixel connecting line 58 overlaps the common line 60, as shown in FIG. 3, so that an overlapped area becomes a storage capacitor CST. More particularly, the pixel connecting line 58 acts as one electrode of the storage capacitor CST, while the overlapped portion of the common line 60 acts as the other electrode of the storage capacitor CST. One of the pixel electrodes 56 of the pixel is connected to a drain electrode 62 so that all of the pixel electrodes 56 can electrically communicate with the TFT T. However, the IPS-LCD device having the above-mentioned multi domains has a problem of color shift depending on the viewing angles, because the liquid crystal molecules have long and thin shapes.
FIG. 4 is a graph illustrating a viewing angle of the IPS-LCD device having the zigzag structure shown in FIG. 3. The IPS-LCD device having the zigzag-shaped common electrode and pixel electrode can have an improved viewing angles in the directions of ±90 and ±180 degrees, i.e., in right-and-left and up-and-down directions, as illustrated by references “IVa” and “IVb” in FIG. 4. However, the viewing angles are degraded in the directions of ±45 and ±135 degrees, i.e., in diagonal directions, as illustrated by references “IVc” and “IVd” in FIG. 4. Further, color shift also occurs depending on the viewing angles or directions.
When the voltages applied across the common electrode and pixel electrode generate the electric fields between the common electrode and pixel electrode, the liquid crystal molecules rotate about 45 degrees to re-align in accordance with the electric fields. Gray inversion can occur due to the rotation of the liquid crystal molecules. Especially, when the IPS-LCD is operated in gray mode, the IPS-LCD produces yellowish color shift in 45(+45) degrees declination with respect to the liquid crystal polarization because of the optical anisotropy properties of liquid crystal molecules. In addition, a bluish color shift in 135(−45) degrees declination with respect to the liquid crystal polarization can occur because of the optical anisotropy properties of the liquid crystal molecules.